Magnetic pick-up circuits, in the general sense, are not new. For years, different applications have demanded detection of the velocity or position of rotating shafts. The most efficient and reliable way to detect the velocity of a rotating shaft is to use the effect of magnetic induction rather than by sensing shaft motion directly by mechanical means.
Early on, where mechanical means were employed to detect shaft velocity, it was realized that mechanical variations between elements of the mechanical means could give rise to false readings as to shaft velocity. Therefore, prior to art devices incorporated various schemes to circumvent the problem of mechanical variations. Magnetic pick-up arrangements appear to provide an answer to problems of this nature. Magnetic pick-up arrangements involved in the detection of speed or shaft position addressed some common technical concerns.
Typical of the prior art is the Sato et al. patent 4,369,405 which describes a rotational position detecting apparatus. A rotating shaft is fitted with a plurality of magnetic poles on the periphery thereof except at a portion thereof indicative of a rotational reference position. A pair of magnetically sensitive means having a mounting pitch therebetween related to the pitch of the magnetic poles is positioned to face the rotating magnetic poles so as to generate an output whose AC components are different in phase by one-half period from each other. A waveform shaping circuit including a differential circuit generates an output indicative of the difference between the output AC components of the pair of magnetic sensitive means. Further, a comparator compares the differential output of the differential circuit with a predetermined level to generate a rectangular wave output. The effect of this combination is that mechanically induced variations affecting the rotating magnetic poles cancel each other. However, Sato et al. require a rotational reference in order to overcome a totally symmetrical cancellation of the signals produced by the pair of magnetic sensitive means. Sato et al. do not entertain the presence of an ambient electromagnetic field as does the invention to be described herein. Further, the rotating magnetic field of Sato et al. would be deleterious to a proximate generator's electromagnetic field due to the electromagnetic interference such a rotating field would create.
The Presley U.S. Pat. No. 3,984,713 is directed to a magnetic speed sensor with a compensating pole. Presley requires the presence of a tone wheel driven by a vehicle and subject to axial misalignment and vibration. The tone wheel is made of magnetic material and has a continuous portion and a toothed portion. Sensing means is placed proximate to the tone wheel and consists of a sensing pole at one side of the tone wheel proximate to the toothed portion, and a compensating pole at the same side of the tone wheel proximate the continuous portion and remote from the toothed portion. In this manner, mechanical vibration induced in the tone wheel results in equal but opposite electric signals being induced in each of the sensing pole and compensating pole. The signals from these poles are added together to cancel each other out, leaving only the signal from the sensing pole produced by the toothed portion of the tone wheel. This signal is, of course, indicative of the velocity of the tone wheel. The invention of the Presley patent, like that of Sato et al., does not concern itself with the presence of an ambient electromagnetic field as in the invention to be described.
The Lokkart U.S. Pat. No. 3,961,214 is directed to a velocity pick-up and employs a similar scheme for cancellation of undesirable signals caused by mechanical vibration or other nonrotational stimulus. Lokkart requires a member rotatable about an axis, which member is mounted to a base and a magnetic rotor fixed relative to the member in a position to move therewith continuously past pick-ups. The rotor is fitted with a set of equally and angularly spaced teeth separated by valleys. A pair of magnetic pick-ups is placed proximate to the rotor, the ends of the pick-up closest to the teeth being spaced apart in the direction of movement thereof. The spacing of the pick-ups is related to the spacing of the teeth. Cancellation of the undesirable signals is accomplished by this spacing scheme, which causes signals produced by the pick-ups not due to rotation of the rotatable member to be cancelled due to their equal but opposite effect on each of the pick-ups. The presence of an ambient electromagnetic field is not a part of the environment in which the Lokkart invention is to function as is the case with the invention to be described.
Other prior art has merely applied the idea of providing magnetic pick-ups to other, less germaine situations.
The Hewett U.S. Pat. No. 4,647,892 is directed to an electronic speedometer which includes a dual magnetic sensor that requires an elongated cylindrical bobbin secured within a magnet and a pair of coils coaxially wound upon and along the length of the bobbin so as to sense equally and independently any disturbance of a magnetic field. In Hewett, the coils act entirely independently; there is no noise cancellation, merely symmetry. More importantly, there is no concern regarding an ambient electromagnetic field.
The Dron et al. U.S. Pat. No. 4,276,489 is directed to a multiple coil electromagnetic pick-up speed measuring device which requires two assemblies, each comprising an axial arrangement of a coil in a straight magnet, arranged end to end, only the coil of each assembly being housed in a solid piece of magnetic material. Dron follows the same line of thought as Hewett. Dron goes one step further by magnetically insulating two magnetic pick-up assemblies and packaging them in one housing. Again, magnetic symmetry is present, but ambient electromagnetic field cancellation is not.
None of the aforementioned inventions were designed to overcome the problem of interference due to operation within an ambient electromagnetic field. Furthermore, these inventions never reduced interference levels by parallel connection of dual pick-up coils as will be described hereinafter. Finally, none of these inventions contemplated operating in close proximity to a generator where their creation of moving magnetic fields could harm the generator's field. Applicant's invention is the first to address the problem of cancelling a signal produced by an ambient electromagnetic field while preserving the ambient electromagnetic field and a signal produced by a modulated flux field.